The present invention relates generally to radio antennas and more particularly to a circularly polarized antenna which is constructed such that its phase center can be very easily and very accurately positioned with respect to a point on the ground or other mounting surface over which the antenna is placed. The antenna can be reproduced with substantially identical mechanical and electrical characteristics. The antenna is especially suited for use in radio interferometry systems for measuring the relative position vector between a pair of survey marks by simultaneous observations at each survey mark of signals transmitted from earth orbiting satellites of the NAVSTAR Global Positioning Systems (GPS) but is not essentially limited to that particular application or to signals received from such sources.
Antennas for receiving or transmitting circularly polarized RF signals are well known in the art and the use of such antennas in radio frequency interferometry systems for measuring the baseline vector between points on the ground using circularly polarized RF signals transmitted from remote locations is also well known in the art.
In U.S. Pat. No. 4,170,776, issued to Robert A. Frosch as Administrator of the National Aeronautics and Space Admininstration with respect to an invention of Peter F. MacDoran there is described a system for use in detecting earth crustal deformation using an RF interferometer technique for such purposes as earthquake predictive research and eventual operational predictions. In the system described in the patent, circularly polarized RF signals from transmitters on the moon or from a plurality of earth orbiting satellites are received at two locations on earth through an antenna situated at each location, and a precise measurement is made of the time dependent phase difference between the signals received at the two locations to determine two or three spatial parameters of the antennas' relative positions. The received signal at each location is precisely time tagged and land-line routed to a central station for real-time phase comparison and analysis. By monitoring the antennas' relative positions, crustal deformation of the earth may be detected. The system includes a circularly polarized "dish" type antenna at each one of the locations from which the observations are made. This type of antenna is highly directional and must be turned toward the source of the signal being observed. Although it is difficult to determine the position of the phase center of this type of antenna, because the antenna is always rotated to point toward the source it follows that the geometrical point to which the interferometric determination of antenna position refers is defined by the two rotational axes about which the antenna turns. Because this type of antenna and the machinery required for turning it are large, the geometrical reference point usually is separated by some distance from the geodetic survey marker on the ground to which position reference must ultimately be made. Considerable effort may therefore be required to determine accurately the relative position vector between the reference point on the antenna and the one on the ground.
In a paper presented at the 9th GEOP Conference, Oct. 2-5, 1978, and appearing in the Department Geodetic Science Report No. 280 pp. 65-85, the Ohio State University, Columbus, Ohio, and in an article by Charles C. Counselman III and Irwin I. Shapiro appearing in Bulletin Geodesique, vol. 53 (1979) pp. 139-163, which articles are incorporated herein by reference, there is described a proposed system for measuring relative position vectors by radio interferometry using circularly polarized Rf signals emitted by earth orbiting satellites over a range of frequencies spanning about an octave in the 1 to 2 GHZ region for use in application such as earthquake monitoring, land surveying, navigation, spacecraft tracking and measuring gravity anomalies. The system features a miniature sized interferometer terminal containing a circularly polarized RF antenna consisting of a stack of crossed pairs of horizontal, half wavelength dipoles made of metal rod or tubing, with each pair of dipoles being cut for one or more of the frequency bands of the signals emitted by the satellites. Each dipole pair is placed three-eighths of a wavelength above a 30 to 90 centimeter diameter metal ground plane and the two orthogonal dipoles comprising each pair are fed in phase quadrature for circular polarization. This type of antenna is not very directional so that ordinarily it would not be turned but would be left fixed in orientation while it received signals from sources in various directions. The geometrical point whose position would be determined by this radio interferometry system is the phase center of the antenna. Although the article and the paper both describe the overall proposed system and certain characteristics of the receiving antennas in considerable detail no mention is made in either the article or the paper as to how the phase center of antenna may be accurately positioned with respect to a survey mark on the ground.
As can be appreciated, in order to accurately measure vectors between survey marks on the ground using radio interferometry systems it is essential that the antennas employed in such systems be constructed so that they can be accurately positioned over the survey marks. It is also essential that the antennas be constructed such that they can be reproduced with substantially identical mechanical and electrical characteristics so that their positioning relative to a survey mark will not vary from unit to unit.
The present invention provides such an antenna.
Accordingly, it is an object of this invention to provide a new and improved antenna.
It is another object of this invention to provide a new and improved circularly polarized antenna.
It is another object of this invention to provide an antenna whose phase center can be very easily and very accurately positioned with respect to a point on the ground or other surface over which it is placed.
It is a further object of this invention to provide a circularly polarized receiving antenna for use in determining relative position vectors between geodetic survey markers by radio interferometry techniques.
It is still another object of this invention to provide an antenna as described above which is particularly suited for use in measuring vectors between survey markers using circularly polarized RF signals emitted by earth orbiting satellites.
It is still another object of this invention to provide an antenna as described above whose phase center can be located with respect to a survey mark to an accuracy of a few millimeters.
It is a further object of this inventon to provide an antenna as described above, that is rugged in design, that is portable, that is relatively inexpensive to fabricate, that is easy to use and position, that can be mass produced in a reproducible manner, that contains a minimum number of parts and that can be easily assembled and disassembled.
It is yet still another object of this invention to provide an antenna as described above which has little directivity, which is capable of receiving signals simultaneously from widely separated directions in the sky with elevation angles as low as 10.degree. and wherein the phase of the signals received is not altered significantly by signals that are reflected or scattered from the ground or other objects near the antenna.
It is another object of this invention to provide a combining network for combining into a single output RF signals received by a set of four antenna elements extending radially outward from a common axis in different directions at 90.degree. intervals.